Abstract We have characterized a novel process carried out by phagocytes that we have called digestive exophagy. We have shown that macrophages and dendritic cells create extracellular, acidified lysosomal compartments to digest large objects such as aggregates of LDL or dead adipocytes that are too large to be phagocytosed. We have found that genetic mutations associated with neurodegenerative disorders (e.g., frontotemporal dementia and Alzheimer's disease) also affect the digestive exophagy of aggregated LDL (agLDL), such as that found in atherosclerotic plaques. This is consistent with similar cellular mechanisms being involved in both cases. In this supplement, we propose to apply the methods we have developed to study digestive exophagy of amyloid A? by microglia. We will also continue to examine mutants identified in neurodegeneration studies for their effect on digestion of agLDL by macrophages. We have reported that macrophages lacking progranulin (GRN) have greatly increased exophagy of aggregated LDL. Mutations or reduced expression of GRN are linked to AD as well as frontotemporal dementia. TLR4 and other signaling molecules are required for digestive exophagy, and these same signaling pathways are modulated by TREM2, a protein that is highly expressed in microglia. Mutations in TREM2 are strongly linked to increased susceptibility for AD. Preliminary studies have shown that microglia create similar acidic compartments upon contact with large aggregates of fA?, and they secrete lysosomal contents into them. In Aim 1 we will characterize the mechanism by which microglia degrade large aggregates of fA?. We expect to confirm preliminary observations of lysosome secretion and compartment acidification. We will use fluorescently labeled fA? to observe and quantify the degradation of fA? deposits. These studies will be carried out using primary mouse microglia. In Aim 2 we will analyze signaling mechanisms that regulate lysosome secretion and degradation of amyloid A? by microglia. Interestingly, preliminary data have shown that lack of Dap12, an effector of TREM2 signaling, in bone marrow macrophages leads to a large increase in lysosome secretion upon contact with LDL aggregates or A? deposits. Similar studies will be carried out in microglia from wild type and knockout mice contacting A? deposits to determine if there is a role for GRN, Trem2, Dap12, Tlr4 and other proteins. As illustrated by the Dap12 results, proteins identified in the Alzheimer's studies may reveal new signaling in macrophages to be studied in the parent grant.